Controller for CNC-operated machine tools

ABSTRACT

A controller for optimization of metal-working on CNC-operated machine tools, includes a first unit for monitoring the torque of the main drive of the machine tool to establish the actual, instantaneous cutting torque, a second unit for setting the rated cutting torque in the teaching mode in dependence on the main-drive torque as monitored, a third unit for calculating the feed rate required to maintain the cutting torque at a constant level and controlling the feed drive, and a fourth unit responsive to the monitored main-drive torque and providing feed rate limiting signals to the third unit for protecting the tool against breakage. The unit for calculating the feed rate is addressed by a compensator unit responsive to signals from a comparator unit comparing the torque as set with the actual, instantaneous torque as indicated by the first unit and to signals from an identifier unit calculating the instantaneous cross- sectional area of the cut in response to signals from both the first, main-drive torque monitoring unit and the feed-rate calculating unit. A method for optimization of metal-working on CNC-operated machine tools is also described.

The present invention relates to a controller and a method foroptimization of metal-working on CNC-operated machine tools, especiallyon CNC-operated milling machines and machining centers.

While CNC-operated machine tools have existed for years, theirefficiency and usefulness has been limited by their incapability to takeinto account many factors in the programming stage which influenceproduction efficiency, including: number of workpieces in a run,operating cost, tool replacement time, tool cost, etc. In addition, therigidly deterministic nature of CNC-operated machine tool programming isincapable of allowing for unforseeable changes in real-time cuttingconditions such as depth and width of metal cutting, tool wear,non-uniformity of workpiece blank, etc.

A recent development in the field of CNC-operated machine tools providesfor apparatus for controlling a machine tool as a function of torqueload on a cutting tool when the torque load respectively exceeds orfalls below a predetermined upper or lower critical torque load. Forexample, U.S. Pat. No. 4,237,408 describes critical torque loadsincluding, inter alia, a catastrophic torque limit relative to themachine structure, a catastrophic torque limit relative to a particulartool and a minimum torque limit that should be present if a cutting toolis in contact with the workpiece.

It is one of the objects of the present invention to overcome thelimitations and disadvantages of today's CNC-operated machine tools andto provide an optimizing controller for machine tools, in particular forCNC-operated milling machines and machining centers, which calculatesthe optimal cutting modes according to production efficiency criteria,and automatically provides adaptive feed and spindle speed controlresponding to real-time cutting conditions, maintains a constant andpresettable spindle torque and/or tool life, ensures optimal machiningoperation, prevents tool breakage and indicates tool status.

According to the invention, this is achieved by providing a controllerfor optimization of metal-working on CNC-operated machine tools, havinga main drive powering the tool spindle of said machine tools and feeddrives powering the feed mechanism of said machine tools, said feeddrives being controllable to produce a feed rate determined either by apredetermined setting of the cutting torque produced by said toolspindle, or by said controller overriding said setting in a teachingmode of said controller, comprising a first unit for monitoring thetorque of the main drive of said machine tool to establish the actual,instantaneous cutting torque; a second unit for setting the ratedcutting torque in said teaching mode in dependence on said main-drivetorque as monitored; a third unit for calculating the feed rate requiredto maintain said cutting torque at a constant level and controlling thefeed drive of said machine tool; a fourth unit responsive to saidmonitored main-drive torque and providing feed rate limiting signals tosaid third unit for protecting the tool against breakage, characterizedin that said unit for calculating said feed rate is addressed by acompensator unit responsive, on the one hand, to signals from acomparator unit comparing said torque as set with the actual,instantaneous torque as indicated by said first unit and, on the otherhand, to signals from an identifier unit calculating the instantaneouscross-sectional area of the cut in response to signals from both saidfirst, main-drive torque monitoring unit and said feed-rate calculatingunit, said compensator unit facilitating a high-precision stabilizationof said torque.

The invention furthermore provides a method for optimization ofmetal-working on CNC-operated machine tools having a main drive poweringthe tool spindle of said machine tools and feed drives powering the feedmechanism of said machine tools, said feed drives being controllable toproduce a feed rate determined by a predetermined setting of the cuttingtorque produced by said tool spindle, or by said controller overridingsaid setting in a teaching mode of said controller, comprising the stepsof monitoring the torque of the main drive of said machine tool toestablish the actual, instantaneous cutting torque; setting the ratedcutting torque in said teaching mode in dependence on said main-drivetorque as monitored; calculating, in a feed rate calculating unit, thefeed rate required to maintain said cutting torque at a constant leveland controlling the feed rate of said machine tool; providing feed ratelimiting signals to a feed rate calculating unit for protecting the toolagainst breakage; comparing, in a comparator unit, said torque as set,with said actual, instantaneous torque; calculating, in an identifierunit, the instantaneous cross-sectional area of the cut in response tosignals produced by both said main-drive torque monitoring unit and saidfeed rate calculating unit; feeding the signals from said two units to acompensator unit, and feeding the signals from said compensator unit tosaid feed rate calculating unit, thereby achieving high-precisionstabilization of said cutting torque.

The invention will now be described in connection with certain preferredembodiments with reference to the following illustrative figures so thatit may be more fully understood.

With specific reference now to the figures in detail, it is stressedthat the particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention, the description taken with the drawings making apparent tothose skilled in the art how the several forms of the invention may beembodied in practice.

In the drawings:

FIG. 1 is a block diagram of a first embodiment of the controlleraccording to the invention;

FIG. 2 is a diagram illustrating the effect, on the feed-rate values andthe torque values, of the compensator unit;

FIG. 3 is a block diagram of a second embodiment of the controlleraccording to the invention; and

FIGS. 4 and 5 illustrate a third and a fourth embodiment, respectively,of the controller according to the invention.

The principal input parameters of the first and second embodiments ofthe controller according to the present invention are one or more of themain-drive parameters which are proportional to the cutting torque M.The principal output parameter is a signal determining the feed rate Fas a function of M, the task fulfilled by the invention being tomaintain this torque at a steady level determined in dependence on theproperties of the specific milling cutter used. The required values canbe found in appropriate tables.

Another concept of the present invention is the teaching mode in which,instead of the maximum rated cutting torque M_(o), a maximum torqueM_(o) ' is determined during the machining of one or several of thefirst identical workpieces. The teaching mode is particularly effectivefor large runs of identical workpieces.

Another important parameter used by the controller according to theinvention is ρ mm² !, designating the cross-sectional area of the cut(for short, area of cut), which is the product of the cut width (b) andcut depth (h).

Referring now to the drawings, there is seen in FIG. 1 a block diagramof a first embodiment of the controller according to the invention,comprising a housing 2 attachable to a CNC-operated milling machine andaccommodating the various units of the controller, and a panel 4 whichis accessible to the operator.

On the panel 4 is located a switch 6 for selecting: initiation of theTeaching Mode (TM) ("initiate"); "Run" for M_(o) settings determined inthe teaching mode, and operation with predetermined M_(o) settings("without TM"). In the latter, the value for M_(o) is set on theselector 8. Other elements on panel 4 include a starting button 10 and atool status indicator 12 which lights up, or provides, e.g., an acousticwarning, when the tool is worn beyond a certain limit.

There is seen a monitoring unit 14 in which the instantaneous main-drivecutting torque M (as applied by the milling cutter) is monitored.

The signal M from the monitoring unit 14 is fed to a number of otherunits of the controller:

a) the unit 16 for setting the rated cutting torque M_(o) forapplication in the teaching mode;

b) a tool protection unit 18 which supplies feed rate limiting signalsto a feed rate calculator 20;

c) a unit 22 for identifying the instantaneous value of ρ, alsoaddressed by the signal from the feed rate calculator 20, and

d) a comparator unit 24 which compares the set torque M_(o) with theactual, instantaneous torque M.

According to the position of the mode switch 6, a logic element 26provides the comparator unit 24 with the M_(o) value as determinedeither by unit 16 or by the manual selector 8.

The controller also includes a self-diagnostic unit 28 interposedbetween the start button 10 on the panel 4 and the feed rate calculator20. When the button 10 is pressed, the unit 28 performs a test of theentire system and, if the latter is found operational, provides anenabling signal to the feed rate calculator 20.

The heart of the controller is constituted by a compensator unit 30 incooperation with the already-mentioned ρ-identifier unit 22.

The following is an explanation of the considerations underlying thecompensation principle.

The feed rate is determined by the difference ΔM between the set valueM_(o) or M_(o) ' and the actual value M.

The metal-cutting process (as static process) can be represented by theexpression:

    M=AF.sup.y ρ.sup.γ

where:

ρ=the already-mentioned area of cut;

F=feed rate, and

A, y, γ=coefficients depending on tool type and metal-workingconditions.

Seeing ΔM as the error of cutting torque stabilization, it can bedefined as: ##EQU1## where: K_(c) =CNC gain (static), and

K₁ =current monitor gain.

However, in real-life machining, ρ<<1/K₁ K_(c) A, as a result of whichΔM≈M_(o), or M≈O, making it impossible to achieve cutting torquestabilization with medium and small ρ-values.

In order to secure for M independence from changes of ρ, it is necessaryto provide a compensator unit with variable gain K_(k) : ##EQU2## with Bbeing a constant.

To calculate K_(k) it is thus necessary to determine ρ at every instantthroughout the cutting process, which is done by unit 22 according tothe assumption that ρ is proportional to the ratio ΔM/F.sup.∝, where ∝is determined for each material to be cut.

The effect of the compensator unit is shown in FIG. 2, in which thesolid curves 32 and 34 indicate the values of F and M/M_(o) as functionsof ρ (specifically, of the cut height h) with compensation, and thedashed curves 36 and 38 indicate the same values F and M/M_(o) withoutcompensation.

The feed rate of the machine tool is obviously controlled by the outputF of the feed rate calculator 20.

FIG. 3 shows another embodiment of the controller according to theinvention. This embodiment differs from the previous embodiment in thatthe controller is inaccessible to the operator, being addressed only bythe CNC program. Added elements in this embodiment are a programinterface 40 linking the controller to the CNC program and a memory unit42 for the rated torque M_(o) of a number of different tools N (asmarked MN₃ -MN₂₅) to be used in the machining process, with MN₀ and MN₁signifying selection of the teaching mode and MN.sub. --without teachingmode. The rest of the unit is identical with the units of the previousembodiment and operate in the same manner.

The embodiment illustrated in the block diagram of FIG. 4 is intendedfor the optimization of machining operation on the basis of either oneor the other of two criteria:

1) maximum metal removal per unit time (mm³ /min);

2) minimum cost of removal of unit volume of metal ($/min).

It is possible to select a compromise between these criteria.

The embodiment of FIG. 4 comprises all the units described in connectionwith FIGS. 1 and 3 (except for the panel 4 and its elements), as well assome additional units to be described further below.

While the first criterion is taken care of by the "F-loop" comprised ofunits 20, 22, 24 and 30 (FIGS. 1 and 3) and is conditional upon M=M_(o),the second criterion requires the introduction of an additional unit,44, which constitutes the operative part of an "S-loop", inasmuch as itis meant to control the speed IS) of the tool spindle. This unitconsists of a calculator 44, which realizes the expression: ##EQU3##where: A₃ =coefficient dependent on the specific tool used;

∝₃, ∝₄, ∝₅ =coefficients depending on the material machined;

ρ32 area of cut, supplied by the identifier unit 22,

F=feed rate, and

T_(o) =tool service life required for selected optimization criteria.

The first criterion is conditional upon the relationship: ##EQU4##

The second criterion is conditional upon the relationship: ##EQU5##where: m=coefficient depending on the specific tool used and materialmachined;

τ=auxiliary or idle time (min);

D=cost of tool ($);

B=cost of machining per min ($/min).

The calculator 44 has five inputs:

a) coefficients A₃ for the tools N3-N25 (from memory 46 addressed byinput MN₃ -MN₂₅); p1 b) coefficients ∝₃, ∝₄, ∝₅ for four differentgroups of materials (from memory 48 addressed by input MN26-MN28);

c) signal F (from calculator unit 20);

d) area of cut ρ (from the identifier unit 22), and

e) projected tool service life T_(o) (from unit for calculation ofT_(o)).

Input MN_(o) initiates the teaching mode and input MN₁ runs the teachingmode for all tool diameters.

The outputs of the controller of this embodiment are the same as withthe previous embodiment (tool status and feed rate control signal F),with the addition of the speed control signal S.

The embodiment represented in FIG. 5 has all the features described inthe previous three embodiments, with the addition of two furtherfeatures, namely, a circuit suppressing machine tool vibrations andchatter, and a circuit facilitating the finish machining, at highprecision, of thin wall sections of workpieces.

The first of these features comprises a vibration analyzer 50 addressedby any suitable transducer 51 responding to vibrations and chatter ofthe machine. The output of the transducer 51 is analyzed by unit 50,which produces a signal fed to the feed rate calculator 20 which, inresponse, modifies the feed rate F to the degree required to suppressthe vibrations, returning it to the original rate once this has beenachieved.

The problem with thin sections is their elastic deformability under thecutting pressure of the milling cutter. Thus milling an aluminum wall ofa thickness of, e.g., 2.5 mm and a length of 200 mm, taking a cut of adepth of 0.5 mm at a feed rate of 500 mm/min, a cutter speed of 1000 rpmand a tool diameter of 12 mm, will produce an error of 0.04 mm, whilemilling a section of a thickness of 10 mm at identical cut depth, feedrate, speed and tool will produce an error of only 0.005 mm. Thisdifference is, of course, due to the "giving in", and subsequentspring-back, of the thin section, necessitating a reduction of the feedrate when the milling cutter arrives at such a thin section.

This not only complicates the CNC-program, but it is also difficult todetermine at which point, after a heavy section, the thin sectioneffectively begins. Also, a worn cutter will increase the deformingforce which, with a new cutter, would be much smaller.

It is the task of the present embodiment to automatically reduce thefeed rate the moment wall deformation is detected.

It was found that certain harmonics of the feed-drive current arereduced during the milling of thin walls, due to the change of frequencycharacteristics of the electrical-mechanical loop of which the thinsection is a part. Thus, based on a dispersive analysis of feed-drivecurrent signals, it is possible to form special signals indicating theeffective beginning and ending of a thin section. These signals are usedto reduce the feed rate during the machining of such thin sections, thusincreasing the accuracy of the machining operation.

The added circuit of the embodiment of FIG. 5 comprises a suitablesensor 52 responsive to the feed-drive current, feeding an analyzer 54for analyzing the harmonics of the feed-drive current, which analyzeraddresses a signal transducer 56 producing signals that, fed to the feedrate calculator 20, modify the output signal of the latter, reducing thefeed rate whenever the sensor 52 and analyzer 54 indicate the effectivebeginning of a thin section, and restoring the previous feed rate whenthe sensor 52 and analyzer 54 indicate the ending of this section. Theembodiment of FIG. 3 is particularly suitable for CNC- operatedmachining centers using a pre-programmed sequence of different tools,and is more efficient than the previous embodiment, particularly due tothe provision, as shown in FIG. 3, of the memory unit 42 whicheliminates the need to reset the controller each time a tool is changed.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrated embodiments and thatthe present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed is:
 1. A system for adaptively controlling a feed rate Fof a milling cutter relative to a workpiece, the milling cutterconstituting part of a machine tool having a main drive, the systemcomprising:(a) a torque monitor for monitoring an actual main drivecutting torque M; (b) a torque comparator for calculating ΔM where ΔM=M₀-M and M₀ is a predetermined reference main drive cutting torqueestablished for the milling cutter and the workpiece material; and (c) afeed rate controller for determining the feed rate F as a function ofΔM; wherein said feed rate controller includes means to calculate aninstantaneous cross-sectional area ρ of a cut of the workpiece beingworked on by the milling cutter and determines the feed rate F as afunction of ρ to substantially stabilized M such that ΔM→0.
 2. Thesystem according to claim 1 wherein said feed rate controller calculatessaid cross-sectional area ρ from the general relationship M=AF^(y)ρ.sup.γ where A,y and γ are coefficients dependent on the milling cutterand the workpiece material.
 3. The system according to claim 1 furthercomprising a spindle speed controller for adaptively controlling thespindle speed of the milling cutter to obtain a desired predeterminedtool service life T₀.
 4. The system according to claim 1 furthercomprising a vibration suppression unit for minimizing vibrations of themilling cutter below a predetermined threshold.
 5. The system accordingto claim 1 further comprising a feed drive current analyzer for reducingthe feed rate F during stock removal along a thin walled workpiecesection.
 6. A method for adaptively controlling a feed rate F of amilling cutter relative to a workpiece, the milling cutter constitutingpart of a machine tool having a main drive, the method comprising thesteps of:(a) monitoring an actual main drive cutting torque M; (b)calculating ΔM where ΔM=M₀ -M and where M₀ is a predetermined referencemain drive cutting torque established for the milling cutter and theworkpiece material; and (c) determining the feed rate F as a function ofΔM; wherein step (c) includes calculating an instantaneouscross-sectional area ρ of a cut of the workpiece being worked on by themilling cutter and determining the feed rate F as a function of ρ tosubstantially stabilized M such that ΔM→0.
 7. A method according toclaim 6 wherein the step of determining the feed rate includescalculating the cross-sectional area ρ from the general relationshipM=AF^(y) ρ.sup.γ where A,y and γ are coefficients dependent on themmilling cutter and the workpiece material.
 8. The method according toclaim 6 further comprising the step of:(d) adaptively controlling thespindle speed of the milling cutter to obtain a desired predeterminedtool service life T₀.
 9. The method according to claim 6 and furthercomprising the steps of:(e) monitoring the vibrations of the millingcutter; (f) comparing said vibrations to a predetermined threshold; (g)modifying the feed rate to substantially suppress said vibrations belowthe predetermined threshold; and (h) restoring the feed rate to itsoriginal value for as long as said vibrations are below thepredetermined threshold.
 10. The method according to claim 6 furthercomprising the steps of:(i) monitoring the feed drive current of themilling cutter; (j) analyzing the feed drive current for reducedharmonic levels indicative of stock removal along a thin walledworkpiece section; and (k) reducing the feed rate on the detection ofsaid reduced harmonic levels.